Due to the need to optimise performance of power turbines in gas turbine engines, a nozzle vane array which directs a working fluid onto the power turbine rotor blades should have the capability of varying its nozzle area. This can be achieved by pivoting the vanes in unison about axes extending radially of the turbine. By this means, the total throat area of the nozzle can be varied between maximum and minimum scheduled values during normal operation of the engine.
In an emergency, such as turbine shaft breakage, leading to overspeed of the power turbine, it is highly desirable to have the capability of effecting substantially total obturation (i.e., shut-down) of the nozzle. This action substantially prevents the working fluid impinging on the turbine blades, thereby avoiding freewheeling runaway of the rotating parts of the turbine and consequent failure due to excessive centrifugally induced stresses.
During assembly of the variable area nozzle, it is vital that the vanes and their actuating mechanism are set up so that when the vanes are pivoted to vary the total nozzle exit area during normal operation, the correct vane angles are selected to ensure that variations in nozzle exit flow area do not occur circumferentially around the array of vanes; i.e., the throat areas between adjacent vanes should be substantially identical for all vane pairs. If incorrect vane angles are selected, there may be unacceptable deviation from expected power turbine performance, or even damage to the turbine if the vane angles are very incorrect.